If you want to design the perfect weed, start with a blueprint of Palmer amaranth pigweed. According to North Carolina weed scientist Alan York, “It has very efficient carbon fixation, it is water-use efficient, and when it is hot and dry and crops are struggling to hang on, it’s very happy. It grows an inch or two a day. When it comes into a field, if you aren’t careful, it’s going to become the predominant weed.”
That is exactly what has happened in parts of the Southeast over the last few years, added Bob Nichols with Cotton Incorporated. Nichols and York were speaking at Cotton Incorporated’s Crop Management Seminar, in Tunica, Miss.
“The epicenter of glyphosate-resistant Palmer pigweed is Macon County, Ga. That site is now 70 percent to 80 percent resistant and over 10,000 acres were abandoned in 2007.”
Palmer amaranth is suspected to be resistant on 300,000 acres in 20 counties in Georgia; 130,000 acres in nine counties in South Carolina; 200,000 acres in 22 counties in North Carolina.
To keep the problem from getting worse, growers “have to get serious about resistance management,” York said. “We have to focus on reducing selection pressure.
“We can no longer go with glyphosate-only programs. We have to do something else. Basically, it’s getting more herbicides and more modes of action out there. We’re talking about putting out residuals, tank mixes, full use rates, and if cultivation fits, fine.
“Roundup Ready technology is convenient, easy and forgiving. It made our growers into weed sprayers. Resistant weeds are going to cause us to go back to being weed managers.”
One of the most important strategies for managing Palmer amaranth is early detection, York says. “If we can catch them when they’re in a little spot in the field, that’s a whole lot better than waiting until the whole farm is wrapped up.”
But once they get established in cotton, look out. York says in cotton, there are no salvage post options. “We spent three years looking at everything you could possibly put over-the-top of cotton. The bottom line is that if you have glyphosate-resistant Palmer pigweed, you have to get it pre-emergence or you’re not going to get it.”
There are a few more post options in corn and soybeans, notes York, “but our growers are going to have to be a lot more timely in their applications than they have in recent years. But we also have a lot of ALS resistance in Palmer amaranth and we can’t depend too much on that chemistry. We’re also putting a lot of pressure on PPO inhibitors like Reflex, Valor, Blazer, Flexstar, and others. We cannot afford to lose those chemistries.”
York says in Southeast cotton fields where glyphosate-resistance has not been found, he recommends Cotoran, Direx or Prowl pre-emergence or Valor preplant followed by a banded pre-emergence herbicide. “We recommend a pre-emergence herbicide following Valor preplant because most of our cotton in the Southeast is strip-till. If you run a strip-till rig after Valor, you will lose your control where you tilled. So you may see a band of pigweed where the strip-till ran.”
Post-emergence in cotton where glyphosate resistance has not been found, York recommends either glyphosate alone or with Dual. “At layby, we’d like to see a residual in there, too (Roundup or MSMA plus Direx, Layby Pro, Suprend or Valor). “We encourage growers to limit PPO inhibitors to one application per year, and we’re trying to not depend on ALS materials any more than we have to.
“Where we already have glyphosate resistance, but we don’t have ALS resistance, we recommend a pre-emergence tank mix of Direx and Reflex, Direx and Staple or Reflex and Staple. Or we can fall apply Valor followed by Direx and Prowl pre-emergence.
At post-emergence, if there is no Palmer amaranth, “we’re going with Dual and Roundup. If we have Palmer amaranth up, we’re putting Staple with Dual.
“Where we have glyphosate and ALS resistance, we have fewer options. We’re looking at Direx and Reflex or Prowl and Reflex pre-emergence or Valor preplant followed by Direx and Prowl pre-emergence.”
This can be followed by a post-emergence application of Roundup and Dual which should be applied before Palmer emergence. At layby, apply MSMA with Direx, Layby Pro or Valor.
“The catch is you have to get those pre-emergence herbicides activated in a timely manner or you’re going to pick it with a bushhog. I like to lean toward Valor preplant because you have it out there several weeks ahead of time and you increase the chances that you’re going to get some rain to activate it.”
York notes that Reflex pre-emergence can be applied on the sandy Coastal Plain soils of the Southeast without problems. On heavier soils of the Mid-South, splash-up can be a problem, leading to injury. Syngenta is working on a label for Reflex preplant to get around that concern. York noted that in corn, atrazine-based programs do a very good job on Palmer amaranth. “However, in the Southeast, we have our worst problems with Palmer amaranth on the lighter soils, and unless you’re irrigating, corn is not a consideration.”
With soybeans, “we routinely recommend something preplant or pre-emergence and post tank mixes as needed. The thing we need to get our soybean growers accustomed to is being more timely with applications. Of the post materials we can put on soybeans, most labels specify a maximum of 4-inch Palmer amaranth.
“There’s no question, we have a lot of problems in the Southeast,” York said. “For us, the horse is already out of the barn. For the Mid-South, you don’t want to go down this path we’re on right now.”
While York and Nichols are concerned about glyphosate resistance, growers should keep in mind that resistance to ALS herbicides such as Classic and Harmony is common and the potential to select for resistance to PPO-inhibiting herbicides such as Reflex, Valor, and Cobra certainly exists.
“When we hear the term herbicide resistance, we automatically think glyphosate,” York said. “I don’t want to belittle glyphosate resistance. We know how much we depend on glyphosate, and it is a serious matter. But resistance is not unique to glyphosate.”
Nichols describes the Palmer pigweed situation as a “serious economic threat to weed management in cotton. We need a general resistance management plan for the PPOs. PPOs are between us and the ability to do economic weed control in cotton and soybeans.”
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